Cache Reference Directories

[InsertCreativityHere]

Our current logic is as follows:
Whenever a user changes a file, we trigger a compilation:

• Step 1 is converting the root_uri to a root_path
• Step 2, resolve any user-configured reference directories (relative to root_path)
• Step 3, construct a new SliceOptions with those references
• Step 4, perform the compilation
• Step 5, store the SliceOptions we just created and the CompilationState(Result) returned by slicec in SharedState.

---

This PR makes the following improvements:

• Step 1 only needs to happen once. We only set the root_uri at initialization, and the only thing we do with it is convert it into a root_path. Now, we do this conversion once, on server start-up, and store the root_path instead of root_uri.
• Steps 2 and 3 only need to happen when configuration changes, not every time we re-compile.
  Now, whenever the config changes, we update the references and cache them up-front.
• We're double-storing the references. Once in SliceConfig, and once in SliceOptions. These are redundant.
  Now we store the SliceOptions directly in SliceConfig. Since the SliceOptions only depends on the config. Nothing else.
  • After this change, SharedState only holds a CompilationState. So there's no reason to have SharedState anymore. We just hold the CompilationState directly.

[externl]

I think there's a race condition here. Another thread could perform a compilation or anything else affecting the compilation_state between caching the current files and performing the compilation.

Maybe open an issue unless you want to fix it here since it's somewhat related.

[InsertCreativityHere]

@externl, just to be clear, are you talking about this case?:

   THREAD 1                THREAD 2
update_config
       .
       .             trigger_compilation
       .
trigger_compilation

[externl]

more specifically

   THREAD 1                                    THREAD 2
modify_state|copy_from_state
       .
       .                                trigger_compilation
       .
trigger_compilation

[ReeceHumphreys]

lgtm